This invention generally relates to an electromagnetic clutch, and more particularly, relates to an arrangement and a method for use with an electromagnetic clutch for securing a pulley to an outer peripheral surface of a rotor.
FIG. 1 illustrates an electromagnetic clutch 20 in accordance with the prior art. A front housing 11 of a refrigerant compressor rotatably supports a drive shaft 12 by means of a needle bearing 13. Needle bearing 13 is disposed in an inner peripheral wall of an opening 14 which is formed at the center of front housing 11. An annular axial projection 15 surrounds drive shaft 12 and projects from the front end surface of front housing 11. A shaft seal assembly 16 is disposed in the cavity formed by annular axial projection 15. Electromagnetic clutch 20 includes a clutch rotor 21, an annular electromagnet 22 and an armature plate 23. Clutch rotor 21 includes a rotor 211 having a U-shaped cross-sectional configuration and a pulley 212 attached at the outer peripheral surface of rotor 211. Clutch rotor 21 is rotatably supported on the outer peripheral surface of annular axial projection 15 through a ball bearing 17. Annular electromagnet 22 is disposed in an annular cavity 211d of rotor 211 and is attached to the front end surface of front housing 11 by an annular member 18. A stopper plate 121, fixed at an outer end of drive shaft 12 by a nut 122, supports an elastic armature plate 23 through a plurality of leaf springs 24.
In such an electromagnetic clutch, clutch rotor 211 rotates in response to a power source (not shown), e.g., an engine of an automobile, through a belt (not shown) which is engaged between a pulley 212 of clutch rotor 21 and the power source. Armature plate 23 is attracted to clutch rotor 21 when electromagnet 22 is energized. As a result, drive shaft 12 is driven by the rotation of the power source.
In general, pulley 212 may be secured to rotor 211 by electron beam welding, CO.sub.2 welding or shrinkage fit by heating.
With electron beam welding, pulley 212 is first mounted on the outer peripheral surface of rotor 211. Then, electron beam 31 is directed on the circumferential surface of the bottom portion of pulley 212 as shown in FIG. 2 so as to weld the pulley to the rotor.
With CO.sub.2 welding, pulley 212 is again first mounted on the outer peripheral surface of rotor 211. Then, the outer lower surface of pulley 212 is welded to the outer peripheral surface of the rotor as shown in FIG. 1.
With shrinkage fit, pulley 212 is first heated. Next, pulley 212 is mounted on the outer peripheral surface of rotor 211 as pulley 212 is being heated so as to undergo thermal expansion as in FIG. 3. Then pulley 212 shrinks by natural cooling to fit rotor 211.
However, each of the above-described methods for securing the pulley to the rotor has drawbacks. With electron beam welding, the spacing between the walls of grooves 213 is narrowed by heat transformation because of the electron beam as shown in FIG. 4. This may generate unsuitable engagement between grooves 213 of pulley 212 and a belt member (not shown) which engages the grooves. This unsuitable emgagement may damage the belt. In addition, the equipment used in electron beam welding is complicated and makes securing the pulley to the rotor difficult. CO.sub.2 welding also requires complicated equipment for securing the pulley to the rotor.
Shrinkage fitting requires a high degree of precision in the dimensions of pulley 212 and rotor 211, especially the inner diameter of pulley 212 and the outer diameter of rotor 211, to effectively secure the pulley to the rotor.